The bond energies of F2, Cl2, Br2 and I2 have been studied computationally using gradient-corrected, relativistic density functional theory with large all-electron basis sets. The bond energies of all four compounds have been broken down into electrostatic, Pauli (exchange) and orbital terms at the equilibrium internuclear separation and as a function of r(X–X) using the Ziegler–Rauk energy decomposition scheme. The comparative weakness of the F2 bond is traced to the electrostatic energy, which is considerably smaller than would be expected on extrapolation of the trend in the I2 → Cl2 data. The small electrostatic term is itself a result of the F–F bond being c. 0.1 Å longer than might be expected, due to the large gradient in the Pauli repulsion around the equilibrium internuclear separation. The weakness of the F2 bond is therefore a combination of quantum mechanical (Pauli) and classical (electrostatic) factors. The present conclusions are placed throughout in the context of previous research.
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